The present invention relates to an optical transmission system and, more particularly, it relates to an improved optical transmission system that allows quick switching of traffic from its active optical switch device to a standby optical switch device in an uninterruptive manner, in fields of optical communication, optical exchangers, and optical networks.
It is important, in providing satisfactory communication service, to keep high reliability of link connection between nodes or between end users in an optical network in the fields of optical communication, optical exchangers, and optical networks that transmit information of a large capacity. Possible link connection faults would include, for example, those to be developed in an optical path and an optical transmission system.
Typical optical transmission systems have two kinds of transmission systems, i.e., an active system (0 system) and a standby system (1 system) in order to improve reliability of the link connection. The active system is switched to the standby system that is in a "hot standby" or "cold standby" state when the active system has a fault. The time required for the switching from the active system to the standby system is not equal to zero. Thus, the service should be interrupted during such switching from the active system to the standby system. Switching from the active system to the standby system without any interruption may be achieved only with an uninterruptive switching. For example, Japanese Patent Laid-Open No. 63-70202 discloses a method for switching an active system to a standby system in an uninterruptive manner in case of a fault in an optical transmission path. This method uses an optical waveguide.
For a fault occurred in optical transmission systems, there are following examples. Japanese Patent Laid-Open No. 8-84116 discloses a technique to provide correct switching by means of distinguishing a fault in an optical transmission path and that in an optical transmission system. Japanese Patent Laid-Open No. 5-316043 discloses a technique to relieve double faults, if any, in an optical transmission system. However, the above-mentioned two techniques requires a non-zero time for switching the active system to the standby system with a certain down time of the system.
A transmission rate for optical signals in recent optical communication has been increased to a Giga bit/s order. In addition, a wavelength multiplexing technique contributes to further increase of a volume of information that is multiplexed and transmitted per unit time via a single optical fiber. Thus, a relative information transmission loss due to the downtime has been increased. Such a circumstance results in an increased demand toward a technique that allows quick switching from the active system to the standby system in uninterruptive manner.
Optical switch devices such as optical cross-connect devices, which are a kind of optical transmission systems, can switch optical paths by means of changing a linkage between optical fibers. The optical switch device of this type has a superior property that can transmit a fast, large-volume signal of the Giga bit/s order. Therefore, it is anticipated considerably that such optical switch devices become commonly available in practice. The optical switch device itself is required to have a high reliability when installed into an optical network. More specifically, it is necessary to ensure that the communication service continues without any deterioration of quality even when the optical switch device has a fault.